Automatic merge from submit-queue (batch tested with PRs 45993, 50293)

[Federation] HPA controller

This PR implements the design listed in https://github.com/kubernetes/community/pull/593.
This is still a work in progress, and needs more unit tests to be added.
I will add the integration tests and e2e tests in a separate PR(s).

@kubernetes/sig-federation-pr-reviews 

**Release note**:

```
Horizontal Pod Autoscaling is now available as an alpha feature in federation. 
It can be used to distribute and scale workload across clusters joined in a federation. 
In its current form, it works only on cpu utilization and the support for other metrics is yet to be built in.
```